In order to improve plant reliability, it is frequently necessary to install several pumps running in parallel to pressurize the liquid withdrawn from the cryogenic distillation column.
For instance, there may be two pumps operating in parallel, each pump being sized to pressurize between 50% and 100% of the total gaseous oxygen flow. Alternatively there may be three pumps operating in parallel, each pump being sized for between 33% and 50% of the total gaseous oxygen flow.
The advantage of these configurations is that, in case of failure of one of the pumps, the total flow of pumped liquid can come back to the 100% value in a very short time thanks to the ramp-up of the pumps remaining in operation, thus ensuring the stability of the production in terms of pressure. This is a key parameter for some down-stream equipment fed by the gaseous oxygen since the equipment may trip if the pressure drop is too large.
In practice, the configurations described above are difficult to implement due to the fact that pump suppliers have trouble manufacturing a pump which can handle such high operating ranges. The pumps have to function during turndown of the air separation unit with all the pumps running but also during full operation of the air separation unit with one pump out of action. In a normal configuration, this situation could be handled by recycling the liquid discharged by the pump so as to build up its load, but in the case where production pumps are located downstream of a storage tank, recycling the pumped liquid to the storage would lead to a critical loss of oxygen molecules by flash, directly impacting the recovery of the air separation unit.